// 用法如下:
function fn1(x) {
  return x + 1;
}
function fn2(x) {
  return x + 2;
}
function fn3(x) {
  return x + 3;
}
function fn4(x) {
  return x + 4;
}
const a = compose(fn1, fn2, fn3, fn4);
console.log(a(1)); // 1+4+3+2+1=11

function compose(...fns) {
  console.log(fns[0](1));
  return function (val) {
    let res = val
    for (let i = 0; i < fns.length; i++) {
      res = fns[i](res)
    }
    return res
  }
}